Johannes Knapp

Simulations for ultra high energy cosmic ray experiments

Ultra-high energy cosmic rays (E >10^19 eV) are recorded since more than 40 years. They pose very fundamental physics questions which are still unanswered today: Where do they come from? What are they? How are they accelerated to these ultra-high energies?
To find answers is difficult because of their extremely low flux (only about 1 particle per km^2 and year) and because charged particles are deflected in cosmic magnetic fields and do, therefore, not point back to their sources.

At those fluxes cosmic rays can only be measured indirectly, i.e. by instrumenting huge natural volumes, such as the atmosphere, sea water or ice, in order to detect the secondary particle showers the cosmic rays induce when interacting. This approach, however, requires to reconstruct the primary particle's energy and mass from the shape and the particle content of the shower.

Usually the reconstruction compares the measured showers with those simulated with an elaborate computer model of the shower development for different energies and primaries. The most crucial part of these simulations are the hadronic and nuclear interactions of the primary and secondary particles with the nuclei of the atmosphere. Since hadronic interactions (unlike electromagnetic ones) cannot be predicted from fundamental theories, and since energies in cosmic ray shower are orders of magnitude higher than what is available at man-made accelerators, the interaction models are phenomenological and are the main source of systematic errors.

This lecture will describe the main features of shower simulation programs, discuss their capabilities and limitations, and explain how they are used to interpret cosmic ray data.